Install a recent TracSpamPlugin and spambayes

The exact versions for these are based on the Django trac installation, which has a well working spam filter: https://github.com/django/code.djangoproject.com/blob/master/requirements.txt
`svn+https://svn.edgewall.org/` appears to work too.

Could you add a comment indicating that this isn't a completely arbitrary revision and is in fact the latest revision at the time of this pull request?